In electrostatography a uniform electrostatic charge is placed on a photoconductive insulating layer, selectively exposed to form a latent image thereon. The resulting latent electrostatic image is developed to provide a viable reproduction of an original by depositing on the latent image a finely divided xerographic marking material referred to in the art as "toner". Toner is normally attracted to those areas of the photoconductive layer which retain a charge, thereby forming a visible toner image corresponding to the electrostatic latent image. The image so produced may be transferred to a support surface or otherwise processed. The image may then be permanently affixed to the support by various conventional fixing methods, such as the application of heat or pressure or use of a solvent. In developing the latent image, the toner may be used alone or in combination with a suitable carrier, and additives, for example charge control agent, flow improvers or the like may be added to the toner.
The toner particles usually comprise a thermoplastic resin mixed with a pigment which is uniformly dispersed in the resin by heating and blending the toner ingredients in a suitable mill. After cooling, the blended mixture is then pulverized to form it into finely divided particles of the desired size range.
A xerographic machine is typically designed to operate with toners having specified triboelectric properties, and the machine has a very narrow triboelectric latitude within which it can operate. For example, if the xerographic machine is designed to operate with toner having a tribo of 15 microcoulombs/gram at a given relative humidity the machine will only operate with toners that have a range of about 13 to about 17 microcoulombs/grams.
When using colored pigments in toners, each type of pigment contributes to the triboelectric characteristics of the final toner. It has been the practice in preparing different color toners for use in a given copying machine to align the triboelectric properties of the toners by the use of charge control additives, so that the toners of different colors each have triboelectric characteristics within the operating range of the machine. Therefore, in order to provide a range of toners having different colors for use in a given copying machine it has been necessary to use different manufacturing techniques for each of the colored toners.
Heretofore, microencapsulation has been used for various purposes in the preparation of toners for electrostatography and in the surface treatment of other finely divided solids. Such purposes include thermal stability, chemical resistance, dispersibility, color retention, light fastness and the like. For example, U.S. Pat. No. 4,758,506 discloses a single component dry pressure fixable toner composition comprising a core mixture encapsulated with a polymeric shell by an interfacial polymerization process. U.S. Pat. No. 4,097,404 discloses a method of encapsulating toners comprising polymerization and coacervation resulting in a copolymer encapsulated in an incompatible shell polymer. Similarly, U.S. Pat. No. 4,626,490 discloses an encapsulated toner comprising a core material of a binder resin and magnetic particles encapsulated within a thin shell material. U.S. Pat. No. 4,803,144 also discloses an electrostatographic toner comprising a pressure fixable core material containing a colorant and magnetizable substance, and a pressure rupturable shell enclosing the core material.
U.S. Pat. No. 4,794,066 discloses a liquid electrostatic developer formed by coating organic pigments with a shell of a polymeric resin and flushing a water-wet pigment presscake of the coated pigment into a non-polar liquid. U.S. Pat. No. 3,904,562 discloses encapsulating organic pigments in a vinyl pyrrolidine polymer and flushing an aqueous presscake of the encapsulated pigment into an oleoresinous organic phase.
U.S. Pat. Nos. 4,421,660 and 4,680,200 each disclose encapsulating pigments by use of an emulsion polymerization process, wherein the pigment particles are dispersed in a water insoluble monomer and emulsified to form very small monomer/pigment droplets, followed by polymerization of the monomer to encapsulate the pigment in the resulting polymer. The resulting encapsulated pigment particles are disclosed as being useful for a number of purposes, including toners.
Although it is known to encapsulate finely divided pigment particles with polymeric resins, in the normal electrographic toner manufacturing process, encapsulated pigments are dried and reground before being melt mixed in a toner binder resin. The resulting dispersion of pigment in the resin is then pulverized and classified to provide toner particles of the desired size.
We have now determined that if a polymer encapsulated pigment is ground prior to being blended into the binder resin of the toner, the polymer coating on the pigment particles becomes broken, so that the surfaces of the pigment particles are exposed. As a result, when such pigments are dispersed in binder resins and the dispersion formed into toners, each type of pigment exerts a different effect on the triboelectric characteristic of the final toner. This results in toners of different colors having a relatively wide range of triboelectric properties from one color toner to the next, which adversely affects their usefulness in electrostatographic copying machines.
We have now found that by encapsulating the pigment particles and maintaining the integrity of the polymer shell the particles can be passivated so that, regardless of the pigment used, toners can be made, without the use of charge control agents, which have triboelectric characteristics which fall within a narrow range as compared to the triboelectric characteristics of toners made when such encapsulation was not employed.